Piezoelectric ceramic compositions have widely been used in piezoelectric ceramic devices, such as a piezoelectric transducer, a piezoelectric filer, a piezoelectric actuator, a piezoelectric transformer, and a piezoelectric buzzer. As piezoelectric ceramic devices are demanded to have small overall sizes, thicknesses, and high performance, various multi-layer structures have been developed.
Conventional piezoelectric ceramic composition is sintered at a high temperature, hence causing a sintering furnace to consume a large electric energy. Further, a piezoelectric ceramic device having the multi-layer structure commonly employs platinum or palladium as material of its internal electrodes, which is expensive and has a high melting point, hence being produced expensively.
A conventional piezoelectric ceramic composition which can be sintered at a comparatively low temperature is disclosed in Japanese Patent Laid-Open Publication No. 9-169566. This composition consists mainly of composite oxides having perovskite structures containing Pb, Zr, and Ti, and is doped with Cu and W. A piezoelectric ceramic device having a multi-layer structure employing this composition can employ less expensive silver-palladium alloy as material of its internal electrode.
While the conventional piezoelectric ceramic composition can be sintered at a low temperature of 1100° C., a device including internal electrodes containing more than 90% of Ag can not be sintered and has unstable characteristics in high-temperature and high-humidity condition.
Another conventional piezoelectric ceramic composition which reduces the content of palladium in internal electrodes and which can be sintered at a lower temperature is disclosed in Japanese Patent Laid-Open Publication No. 10-7458. This composition consists mainly of PbTiO3—PbZrO3—Pb(Mn1/3Nb2/3)—Pb(Co1/3Nb2/3)O3 and is doped with CoO and PbO.
This piezoelectric ceramic composition can be sintered at a low temperature of 930° C., however it has a low, unstable insulation resistance under high-temperature and high-humidity condition.
Among piezoelectric ceramic composition having a perovskite crystalline structure of PbTiO3 and PbZrO3, composition having a ratio A/B exceeds 1.00, where A is the content of Pb and B is the content of Zr and Ti, i.e., non-stoichiometry or a surplus of A can be sintered at a lower temperature than composition having the ratio A/B not greater than 1.00. However, the composition having the ratio A/B exceeding 1.00 has an insulation resistance decrease after the sintering. This composition accordingly disables a predetermined voltage to be applied to the composition, hence having its piezoelectric properties decline. Particularly in high-temperature and high-humidity condition, the composition has the insulation resistance decrease from that at an initial status.